1. Field of the Invention
The present invention relates to a method for fabricating a light emitting device.
2. Description of the Related Art
An organic compound can take a wider variety of structures than an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular-design of an organic compound. Owing to these advantages, electronics which utilize functional organic materials have been attracting attention in recent years.
For example, a solar cell, a light emitting element, an organic transistor, and the like are exemplified as an electronic device utilizing an organic compound as a functional material. These devices utilize electric properties and optical properties of such an organic compound.
In particular, light emitting elements including an organic compound as a light emitting material, which have features such as thinness, lightness, high-speed responses, and DC drive at low voltage, are expected to be applied to next-generation flat panel displays. In particular, advantageously, display devices in which light emitting elements are arranged in matrix are superior in viewing angle and visibility to conventional liquid crystal display devices.
The following is said to be a general light emitting mechanism of a light emitting element: when voltage is applied between a pair of electrodes with an electroluminescent (hereinafter also referred to as EL) layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined at emission centers in the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state. As excited states, a singlet excited state and a triplet excited state are known, and light emission is considered to be possible through either of the excited states.
An EL layer included in a light emitting element has at least a light emitting layer. An EL layer can also have a stacked-layer structure including a hole injecting layer, a hole transporting layer, an electron transporting layer, an electron injecting layer, and/or the like, in addition to a light emitting layer.
EL materials for forming EL layers are broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a low molecular material is often deposited by an evaporation method and a high molecular material by an inkjet method, a spin coating method, or the like.
When the evaporation method is employed to manufacture a full-color display device with red, green, and blue light emitting elements, a shadow mask is provided in contact with a substrate between the substrate and an evaporation source and selective formation of a light emitting element for each color can be achieved through this mask.
However, a shadow mask which is used to manufacture a full-color display device is very thin because of the necessity for precise formation of its opening portion. Thus, if the size of a shadow mask is increased as the size of a substrate is increased, problems may arise in that a shadow mask sags and the size of an opening portion is changed. It is difficult to adopt a means for reinforcing a shadow mask because an extremely fine pattern is formed in a region of the shadow mask corresponding to a pixel portion.
Furthermore, miniaturization of each display pixel pitch is increasingly demanded with higher definition of a display device (with an increase in the number of pixels), and there is a trend toward even thinner shadow masks.
On the other hand, a method for directly forming an EL layer over a deposition substrate by a wet method such as an inkjet method or a spin coating method can also be employed when the size of a substrate is increased; however, it is difficult to make a uniform thickness by this method. When a wet method is employed, it is necessary that a composition or a solution including an EL material is applied and then baked to remove a solvent. Thus, in the case of stacking layers including an EL material, an application step and a baking step should be repeated, and it takes a lot of time. In the case of stacking layers by a wet method such as an inkjet method, a layer should be formed using a solvent in which a previously formed layer is not to be soluble, and there are only limited choices of materials and stacked-layer structures to be used. If there are only limited choices of materials or stacked-layer structures to be used, there is a significant limitation on the performance of a light emitting element (such as emission efficiency or lifetime). Thus, a wet method may cause a major obstacle to improvement in the performance of a light emitting device; for example, it may prevent the application of even a light emitting element with an excellent structure to a light emitting device.